Light Properties Lecture

8/6/2019 Light Properties Lecture

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Properties of lightProperties of light

1. Speed of light is c =2.9979 x 108 m s-1.

c is an absolute constant and nothing

travels faster than c.2. Light has two characteristics. It is a

wave and it can act as particles. This

is known as duality nature of light.3. It is an electromagnetic wave which is

visible to our eyes as 7 colours.


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4. Since it is a wave, the followingcharacteristics of light are observed:

(a) Reflection

(b) Refraction(c) Diffraction

(d) Interference

(e) Polarization5. Light as a wave has the relationship:

c = f where f is frequency,

is wavelength


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5. When light acts as particles, theyare energy particles known asphotons.

6. The energy of photons:

E = nhf

where n is no. of photonsh is Planck constant

f is frequency of light


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LASER

LASER

Light travels in a straight line


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Shadows

Shadows are places where light is blocked:

Rays of light


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Light travels much faster than sound.For examples:

1) Thunder and lightningstart at the same time,but we will see the

lightning first.

2) When a starting pistolis fired we see theflare first and thenhear the bang.


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Light is a visible electromagneticLight is a visible electromagneticwavewaveElectromagnetic SpectrumElectromagnetic Spectrum

Increasing Wavelength

Visible Light

400nm 500nm 600nm 700nm


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Reflection of LightReflection of Light

Reflection from a mirror:

Incident ray

Normal

Reflected ray

Angle ofincidence

Angle ofreflection

Mirror


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The Law of ReflectionThe Law of Reflection

Angleof incidence= Angleof reflectionAngleofincidence= Angleofreflection

Thesame !!!


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We see things because they

reflect light into our eyes:

KKBI


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Luminous and non-luminous objects

A luminous object is one that produces light.

A non-luminous object is one that reflects light.

Luminous objects Reflectors

The sun The moon

Lamps Mirrors


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Clear vs. Diffuse ReflectionClear vs. Diffuse Reflection

Smooth, shiny surfaceshave a clear reflection:

Rough, dullsurfaces havea diffuse reflection.

Diffuse reflection is whenlight is scattered indifferent directions


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Using mirrorsUsing mirrors

Two examples:

1) A periscope

2) A car headlight


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Refraction of Light


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Laws of refractionLaws of refraction

i

r

i

rglass

Vaccum/air1. The incidence ray,

refraction ray andthe normal all lie inthe same plane;

2. nconstant,r

i!

sin

sin

Normal


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Whitelight

Prism

Red

OrangeYellow

Green

Blue

Indigo

Violet

The colours of the rainbow:The colours of the rainbow:


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i

rred


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Wavelengths of coloursWavelengths of colours


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DiffractionDiffraction


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Diffraction of lightDiffraction of light


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a Diffraction pattern of light

Diffraction of lightDiffraction of light

Around a razor bladeAround a razor blade

At a slitAt a slit

Around a circular discAround a circular disc


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Interference of lightInterference of light

lampS1

S2

waves fromS1

waves fromS2

fringesinterference occurswhere beams cross


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Polarization of LightPolarization of Light

Polarization is the separation ofPolarization is the separation ofa beam of light so that thea beam of light so that the

vibrations are in one plane.vibrations are in one plane.It is an exclusive property ofIt is an exclusive property oftransverse waves.transverse waves.

When a light wave is produced, itWhen a light wave is produced, itvibrates in many directions.vibrates in many directions.


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Polarization of LightPolarization of Light


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Photoelectric effectPhotoelectric effect


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Photoelectric effectPhotoelectric effect

Emax

ffo0

-W Emax = hf - W


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Emax = hf W

W is the work function, i.e. theenergy required to release theelectrons;

But Emax = eV ,Then eV = hf W,

When eV = 0, W = hfo eV = hf hfo

eV = h (f fo)


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LensesLenses

f

F

Focal plane


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Angular magnification, MAngular magnification, M

ao

D

ho

D

ha

object,anbyretinatheonsubtendedAngle

oo !

vision)eye(normalcm25Dwhere !


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Angular magnification, MAngular magnification, M

ho a1

u

f

f

u

ha 01 !

u

DM

h

D

u

h

a

aMion,magnificatAngular

0

0

0

1

!@

v!

!@

Magnification of image at

infinity


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M when u = fM when u = f

f

DM !

ho a1 ff

Magnification of image at infinity


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TelescopeTelescope

Eye

lens

Virtual

image

Fe

Fe

Fo focal point ofobject lens

Fe focal point ofeye lens

Fo

Object lens

Realimage


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Magnification of Telescope at NormalMagnification of Telescope at NormalAdjustmentAdjustment

Imageformed at

infinity

Fe

Fe

Fo

fo fe

h a1ao

ao

e

o

o

e

0

1

f

f

hf

fh

a

aM

!

v!

!

e

o

f

fM !

Length of telescope is (fo+fe)


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MicroscopeMicroscope

Virtual

image

Object

Object lens

Eye

lens

FeFo

FeFoRealimage


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Magnification of a microscopeMagnification of a microscope

FeFo

FeFo

ho

h

h1

a1

e0 mmM v!


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D

ha,icroscopeusingretinatheatsubtendedAngle

D

haobject,observetoretinatheatsubtendedAngle

11

00

!

!

0e

0

0

e

e

0

1

1

0

0

1

0

1

M

u

v

u

vh

h

h

h

h

h

h

D

D

h

a

a

M

,icroscopeofionMagnificat

v!@

v!v!

!v!

!

e andmo arethe

linearmagnificationofeyelens andobject

lensrespectively


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Opera telescopeOpera telescope

How does the image form in

the opera telescope?


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Light Properties Lecture

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